Flare apparatus

ABSTRACT

A flare apparatus for burning combustible gases. A flare tip unit comprises an inner member and an outer member defining an annulus therebetween. The annulus defines an annular gas passage through which combustible gas passes. Air moved by a motive force, preferably steam, passes through the inner member and a steam/air mixture exits an outlet of the inner member. The combustible gas and an air/steam mixture mixes in a premix zone between the inner member outlet and the exit opening of the outer member. The combustible gas/air/steam mixture is ignited for burning in the atmosphere above the exit opening. The flare apparatus may include a plurality of flare tip units.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims the benefit of U.S.patent application Ser. No. 11/390,953, filed on Mar. 27, 2006.

BACKGROUND OF THE INVENTION

The present invention relates to an improved flare apparatus and morespecifically to an efficient steam-assisted flare apparatus.

Flare apparatus for burning and disposing of combustible gases are wellknown. Flare apparatus are commonly mounted on flare stacks and arelocated at production, refining, processing plants and the like fordisposing of flammable waste gases or other flammable gas streams whichare diverted for any reason including but not limited to venting,shut-downs, upsets and/or emergencies. Flare apparatus are extremelyimportant in the event of plant emergencies such as fire or powerfailure and a properly operating flare system is a critical component toprevent plant disruption in any of the above-mentioned or othercircumstances.

It is generally desirable that the flammable gas be burned withoutproducing smoke and typically such smokeless or substantially smokelessburning is mandatory. One method for accomplishing smokeless burning isby supplying combustion air with a steam jet pump, which is sometimesreferred to as an eductor. Combustion air insures the flammable gas isfully oxidized to prevent the production of smoke. Thus, steam iscommonly used as a motive force to move air in a flare apparatus. When asufficient amount of combustion air is supplied, and the supplied airmixes well with combustible gas, the steam/air mixture and flammable gascan be smokelessly burned. In a typical flare apparatus, only a fractionof the required combustion air is supplied using motive force such asblower, a jet pump using steam, compressed air or other gas. Most of therequired combustion air is obtained from the ambient atmosphere alongthe length of the flame.

One type of known steam-assisted flare apparatus comprises a generallycylindrical gas tube into which flammable gas is communicated. Lowersteam is communicated through a plurality of steam tubes at an inlet andis forced to negotiate a bend in the steam tube, which causes a pressuredrop. At the bend, the steam tubes are redirected so that they areparallel with the outer cylinder. Center steam is injected into thecenter of the gas tube so that flammable gas and steam pass upwardlythrough the outer tube and is mixed with steam that exits the lowersteam tubes. At the upper end or exit of the gas tube, steam injectorsdirect steam radially inwardly to control the periphery of the mixtureexiting the gas tube, and the steam/air and gas mixture is ignited. Thecenter steam is provided to ensure burning does not occur internally inthe gas tube. Internal burning is typically seen at low gas flow ratessuch as purge rates, and is aggravated by cross wind, capping effectscaused by the upper steam, and if the purge gas has a lower molecularweight than air. A purge rate is typically the minimum gas flow ratecontinuously flowing to the flare to prevent explosion in the flarestack.

Another type of steam-assisted flare uses only center and upper steaminjectors, and works in a similar fashion. The steam-assisted flaresdescribed herein may accomplish smokeless flaring. However, such flareapparatus may create an excessive amount of noise. The noise from thelower steam can be muffled, while the noise from the upper steam isdifficult or impractical to muffle due to its vicinity to the flareflame. A muffler for the lower steam not only adds to the costs, butalso increases the wind load of the flare stack, resulting in increasedflare stack costs. Due to the high cost of steam and the piping andflare stack structure associated with delivering the steam, it isdesirable that less steam be utilized to achieve smokeless burning.Thus, there is a need for an improved flare apparatus and methods forsmokelessly burning combustible gases with air to lessen the noise andto increase the efficiency whereby more fuel may be burned with lessadded steam.

SUMMARY OF THE INVENTION

A flare apparatus in accordance with the current invention includes aplurality of flare tip units. Each flare tip unit has an outer memberwith first and second ends and an inner member defining an inlet and anoutlet. At least a portion of the inner member is disposed andpreferably is coaxially or concentrically disposed in the outer member.An annular gas passage is defined between the inner and outer member ofeach flare tip unit. An upper end of the outer member defines an exitopening while an upper end of the inner member defines the inner memberoutlet. Air passes through the inner member and exits the inner memberoutlet into the outer member.

Combustible gas passes through the annular gas passage and will exit theannular gas passage into the outer member above the inner member outletwhere the combustible gas mixes with at least air in the outer member.The space between the inner member outlet and the exit opening may bereferred as a premix zone, since gas and at least air mix therein priorto exiting through the exit opening for burning in the atmosphere.

While mechanical devices such as fans or blowers may be utilized to moveair through the inner member, preferably steam is utilized as the motiveforce for the air. Likewise, compressed air, nitrogen, carbon dioxide,fuel gas or other gases can be used as a motive force similar to themanner steam is used. In a preferred embodiment of the currentinvention, steam is injected into an inlet of the inner member at a ratesufficient to draw air into the inner member so that a steam and airmixture passes through the inner member outlet into the premix zone.Preferably, the length of the premix zone is greater than the width ofthe annular gas passage and preferably is at least four times the widthof the annular gas passage. The premix zone provides a space for the gasto mix with the air and steam and likewise comprises a perimetercontrol.

In a preferred embodiment, the flare apparatus of the current inventioncomprises a plurality of flare tip units, wherein the annular gaspassage in each of the plurality of flare tip units receives gas from asingle combustible gas supply. The single combustible gas supply may befor example a plenum to which each flare tip unit is connected. Thecombustible gas may be communicated from the plenum into the annular gaspassage of each flare tip unit and a combustible gas and air/steammixture will pass through the exit opening of each of the flare tipunits in to the atmosphere. Each flare tip unit in the plurality ofunits will preferably have a steam injector associated therewith forproviding the motive force for the air through the inner member of theflare tip unit. Steam is preferably provided to each of the steaminjectors from a single source. The combustible gas may be communicatedto the plenum through a gas pipe that will be connected in a flarestack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the flare apparatus of the currentinvention.

FIG. 2 is a section view is a section view taken from lines 2-2 of FIG.1.

FIG. 3 is a section view similar to FIG. 2 of an additional embodimentof the current invention having a generally cylindrical shaped plenum.

FIG. 4 is section view of an embodiment of the invention which utilizesa gas riser as a gas supply.

FIG. 5 is a view looking from line 5-5 of FIG. 4.

FIGS. 6 and 7 are alternative embodiments of flare tip units.

FIGS. 8-14 are alternative embodiments for flare tip units andspecifically embodiments which have different outer memberconfigurations.

FIG. 15 shows an embodiment of a single flare tip unit.

FIGS. 16 and 17 are schematic depictions of a prior art flare apparatus.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, a flare apparatus, which may be referredto as a flare tip 10 is shown. Flare apparatus 10 is adapted to be usedat the top of a flare stack, which as known in the art will communicatea combustible gas from a combustible gas source to flare apparatus 10.The combustible gas may be a waste gas from a refinery, processingplant, chemical plant, production site, LNG production plant, or othersource. The gas may comprise, for example, propane, propylene, naturalgas, hydrogen, carbon monoxide, ethylene or other gas. Flare apparatus10 includes a plurality of flare tip units, or flare structures 15 forreceiving the combustible gas from a single gas supply 20, which in FIG.1 is a plenum 20. A gas pipe 25 connectable to the flare stack (notshown) will deliver combustible gas from the combustible gas source tothe plenum 20.

Flare apparatus 10 may include a plurality of steam injectors 30 forproviding a motive force to move air through each flare tip unit 15.Thus, each flare tip unit 15 may have a steam injector 30 associatedtherewith. Preferably, steam is provided to each steam injector 30 froma single steam source (not shown). The steam source may be connected tothe steam injectors and controlled by any means known in the art. Inoperation, combustible gas is delivered into the plenum 20 through gaspipe 25. An air/steam and combustible gas mixture exits each of theflare tip units 15 and is ignited for efficient burning in theatmosphere. The flare apparatus 10 of the current invention is moreefficient than prior art flare tips in that less steam is required.Apparatus 10 also operates with a lower noise level than othersteam-assisted flare apparatus. These and other advantages will beexplained in more detail hereinbelow.

Referring now to FIG. 2, each flare tip unit 15 comprises an innertubular member 32 and an outer tubular member 34. Inner member 32 ispreferably a generally cylindrical inner member having a longitudinalcentral axis 36. Inner member 32 has first or lower end 38 and second orupper end 40. An inlet bell 42 may be defined at first end 38. The inletbell will direct steam to the inlet 44. Steam injector 30 may be aspider-type injector, wherein the spider arms have holes through whichthe steam is injected. The steam may be directed into the surface of theinlet bell, and may be similar to an internal Coanda nozzle. Innermember inlet 44 is defined at lower end 38, while upper end 40 definesinner member outlet 46. In the preferred embodiment at least air, andpreferably a steam/air mixture will pass through inner member 32 andthrough inner member outlet 46 into outer member 34. Inner member 32 hasouter surface 48 and inner surface 50, which defines a passageway 52 forthe air, or air/steam passing therethrough. Inner member 32 ispreferably a straight cylinder from inlet 44 to outlet 46 with no bends,protrusions, depressions or other interruptions so that the flow of airor steam and air therethrough is uninterrupted.

Outer member 34 is preferably coaxial with inner member 32, and shareslongitudinal central axis 36. Outer member 34 has first or lower end 54and second or upper end 56. An exit opening 58 is defined at upper end56. Outer member 34 has outer surface 60 and inner surface 62. Anannular passageway which may be referred to as an annular gas passage 64is defined by and between inner member 32 and outer member 34. A gasinlet 66 is defined in the embodiment shown at the lower end 54 of outermember 34 and a gas outlet 68 is defined at upper end 40 of inner member32. As is apparent from the drawings, inner member outlet 46 ispositioned lower than and is spaced from exit opening 58. The distancebetween outlet 46 and exit opening 58 may be referred to as a premixzone 70. Combustible gas exiting annular gas passage 64 through gasoutlet 68 will enter the premix zone 70 and will mix with at least air,and in the embodiment shown an air and steam mixture passing throughinner member outlet 46. The combustible gas will mix with the air/steammixture in premix zone 70, and the gas/steam/air mixture will passthrough exit opening 58 and will be ignited for burning in theatmosphere. Thus, the length of the premix zone is such that theair/steam flow in the internal cylinder will expand and mix with thecombustible gas. A length 72 of premix zone 70 is preferably greaterthan a width 74 of annular gas passage 64 and is more preferably atleast four times greater and more preferably four to five times greaterthan the width 74 of annular gas passage 64. The portion of outer member34 that extends above inner member 32 to define premix zone 70 may alsobe referred to as a perimeter control portion since, in addition toallowing air and combustible gas to mix before combustion occurs, thatportion of the outer member prevents ambient wind from sweeping awayunburned combustible gas or causing smoke in the atmosphere.

In a preferred embodiment, outer member 34 comprises a cylindricalsection 78 which extends from lower end 54 of the outer member to anupper end 80 of cylindrical section 78. Cylindrical section 78 may bereferred to as a first cylindrical section 78. A radially inwardlydirected cone, which may be referred to as a convergent cone 82, extendsupwardly from upper end 80 and has an upper end 84. Convergent cone 82will preferably promote mixing between gas and at least air. A secondcylindrical section 86 extends upwardly from convergent cone 82. Secondcylindrical section 86 will further promote mixing between gas and atleast air and allows a more even velocity profile. Second cylindricalsection 86 has an upper end 88. A radially outwardly directed cone whichmay be referred to as a divergent cone 90 extends upwardly from upperend 88. Preferably, divergent cone 90 diverges radially outwardly fromsecond cylindrical section 86 at an angle of about 45° A flame retentionring 92 which is preferably a generally horizontal flame retention ringextends radially inwardly from upper end 91 of divergent cone 90. Flameretention ring 92 may have a plurality of openings 99 which will allowthe combustible mixture to pass therethrough and form a stable flame onflame retention ring 92. FIG. 1 shows eight openings 97. However, therewill preferably be more openings with closer spacing than the spacingshown in FIG. 1. Flame retention ring 92 preferably will not obstruct orlimit flow of the air/steam and combustible gas mixture so that it willnot cause combustible gas to flow backward or downwardly in the innermember in the case where the assisting media or motive gas (i.e., steam,compressed air, fuel gas or any other gas) or blower air is lost. Theinternal diameter of the flame retention ring 92, which comprises exitopening 58, is preferably equal to or only slightly smaller than theinternal diameter of second cylindrical section 86. Preferably, theinternal diameter of flame retention ring 92 is such that exit opening58 has a cross-sectional area no less than the cross-sectional area ofthe annular gas outlet 68, and more preferably 20% more than the area ofgas outlet 68.

In the embodiment of FIG. 2, plenum 20 comprises a generally curvedupper plate 93 and a curved lower plate 94 which in cross section form agenerally oval shape, and which define a plenum interior 95. Outermember 34 extends into plenum interior 95, so that lower end 54 and gasinlet 66 are disposed therein. Outer member 34 may have an inlet bell97. Alternatively, outer member 34 may terminate in lower end 54 atcurved upper plate 93, so that gas inlet 66 may be defined at the curvedupper plate 93. Inner member 34 extends completely through plenum 20, sothat the first and second ends 38 and 40, respectively, are positionedexterior to the plenum 20. Thus, a single combustible gas supply, namelyplenum 20, provides combustible gas to a plurality of flare tip units 15and more specifically communicates gas from a combustible gas source(not shown), which enters plenum 20 through gas pipe 25 to the annulargas passage 64 of each flare tip unit 15.

Combustible gas exits the annular gas passage 64 through gas outlet 68and enters premix zone 70. The combustible gas mixes with at least airthat is moved through inner member 32. Preferably, air is moved througheach inner member 32 with steam that is injected into inner member 32with a steam injector 30. As set forth herein, steam is preferablyprovided to each injector 30 from a single steam source, and is injectedat a rate such that air will be drawn into inner member 32 along withthe steam through inlet 44. Steam injector 30 may comprise a spider-typeinjector, or other known injector, or the steam injector and inlet bell42 may act similar to an internal Coanda nozzle. An air/steam mixturewill pass through inner member outlet 46 into premix zone 70 and mixwith the combustible gas therein. The combustible air/steam mixture willpass through exit opening 58 where it will be ignited and burned in theatmosphere.

Other plenum configurations may be used, and the description herein isnot intended to be limiting. For example, the flare apparatus 10 a shownin FIG. 3 has a plenum 96 that comprises a generally cylindrical drumwith a lower plate 98, upper plate 100 and side wall 102 connecting theupper and lower plates 98 and 100. Like elements of the flare tip unitsare numbered similarly to the flare tip units in FIG. 2, but include thesubscript “a.” Plenum 96 defines a plenum interior 104 to which thecombustible gas is provided as explained with respect to the embodimentshown in FIG. 2. In the embodiment shown in FIG. 3, a molecular seal, ortubular seal 106 is included. Molecular seal 106 has a lower end 108connected to lower plate 98 and extending upwardly therefrom to an upperend 110. Upper end 110 is positioned at an elevation higher than lowerend 54 a of outer member 34 a and circumscribes lower end 54 a, so thata seal annulus 112 is defined between molecular seal 106 and outermember 34 a. Thus, lower end 54 a of outer member 34 a is positionedwith plenum interior 104 in the embodiment shown in FIG. 3. Combustiblegas must pass into plenum 96 and around the upper end 110 of molecularseal 106, around lower end 54 a of outer member 34 a and upwardly intothe annular gas passage 64 a. Molecular seal 106 is optional but may beused to reduce the possibility of any internal burning or purge gasrequirement. Molecular seal 106 will prevent air from moving into theplenum 96 and will prevent burning in the plenum. If air is heavier thanthe combustible gas the air will sit at the bottom of molecular seal106. If air is lighter than the combustible gas, it will be pushed outby the combustible gas.

FIG. 4 shows a flare apparatus 10 b of the current invention, where thegas supply comprises a riser 114 which receives gas from gas pipe 25.Gas riser 114 will distribute gas through tubular spokes 116 which willin turn each communicate combustible gas to flare tip units as describedherein. Flare tip units in FIG. 4 are numbered similarly to FIG. 2, andinclude the subscript “b.”

The flare apparatus of the current invention provides a number ofadvantages over the prior art flare apparatus, one configuration ofwhich is schematically shown in FIGS. 16 and 17. Prior art flare tip 116has an outer cylinder 118 into which combustible gas is communicated.Steam is injected into outer cylinder 118 through a center steaminjector 120. A plurality of lower steam injectors 122 direct steam intoa plurality of lower steam tubes 124. Combustible gas moves in outercylinder 118 between lower steam tubes 124. Upper steam is injectedthrough upper steam injectors 126. Upper steam is necessary to maintainperimeter control and to provide an efficient air/steam and combustiblegas mixture above outer cylinder 118 for smokeless burning.

Flare tip 116 requires more steam than the flare apparatus of thecurrent invention, since steam from the injectors 122 must make bendsand turns rather than following the straight path defined by the innermembers 32 of the current invention. In addition, because of therequired center and upper steam and sometimes lower steam injectors, thenoise generated by the prior art configuration is much greater and mayrequire mufflers for the lower steam. The upper steam is difficult orimpractical to muffle since flare flame can damage these mufflers. Eachflare tip unit of the current invention requires only one injectionlocation for steam and only requires one source of steam while separatesources of steam are typically required for the upper, lower and centersteam injectors in the prior art configuration. Although sometimes thecenter, lower and upper steam can be connected to a common steam line,doing so reduces flexibility of operation and may create problems.

For example, connecting center steam to lower or upper steam renders itimpossible to turn off center steam without turning off the other steamsources that share the common steam line. Under some adverse conditions,it is desirable to turn off the center steam and keep the other steamsources running. These adverse conditions include but are not limitedto 1) freezing or arctic weather, 2) acid gas, 3) gas that reacts withwater to form polymer. Under one or more of the above-mentioned adverseconditions, turning off the center steam typically requires asubstantial increase in purge gas rate to prevent internal burning fromdamaging the flare tip rapidly. The increased purge gas rate oftenrepresents a high cost to the end user. The current invention does notrequire a center steam or a high purge rate to prevent internal burning.Testing has shown that when a minimal amount of motive force (e.g.,steam or blower) is available, internal burning does not occur in theannular gas passage 64 or in the plenum 20, or in pipe 25. In the caseof complete steam failure in the current invention, internal burning canbe prevented, or at least limited by: 1) directing another motive gassuch as compressed air or nitrogen to the steam line; 2) increasing thepurge rate substantially, either of which may be automated.

Another disadvantage of the prior art configuration is the difficulty incoordinating the separate controls of lower and upper steam. Upper steamis typically injected vertically and inwardly. The upper steam fromdifferent steam nozzles may collide at the center above the flare tip,causing a local high pressure zone. This high pressure zone can drive acombustible mixture into the flare tip causing internal burning, anddownward in the lower steam tubes which can cause the whole flare tip tobe engulfed in flame. This is commonly referred to as the capping effectof upper steam. If the lower steam rate is insufficient to overcome thecapping effect, the combustible mixture can travel downward and backwardand exit at the inlet of the lower steam tubes, and the flare tip willbe engulfed in flame causing rapid tip damage. Therefore, it isnecessary to maintain sufficient lower-steam flow rate relative to theupper steam. The current invention requires only one single steamsource, thus eliminating the need to coordinate the control of upper andlower steam.

The flare apparatus of FIGS. 1 and 2 comprises plenum 20 and six flaretip units 15. The riser embodiment of FIG. 4 has four flare tip units.More or less flare tip units may be used in the flare apparatus of thecurrent invention, and if desired a single flare tip unit may beutilized as the flare apparatus. For example, FIG. 15 shows a singleflare tip unit 130. Flare tip unit 130 is similar to each flare tip unit15 and thus has an inner member 132 and outer member 134 defining anannular gas passage 136. Outer member 134 defines an exit opening 138.Inner member 132 is generally identical to the previously describedinner member 32 and will preferably receive steam from a steam injector140 or if desired can simply receive air from a fan or other knownstructure for moving air through inner member 132. It is understood thatinner member 132 may optionally include an inlet bell. In the preferredembodiment, steam will be injected at a rate sufficient to entrain airand move air upwardly therethrough through an outlet 142 at the upperend of inner member 132 and into a premix zone 144. Outer member 134 hasa closed lower end 145, and combustible gas inlet or entry 146 isdefined through the side of outer member 134. Otherwise, outer member134 is substantially identical to previously described outer member 34.Combustible gas will be provided from a flare stack as known in the art.The operation of a single flare tip unit 130 is as described withrespect to flare tip units 15 in that the steam/air and combustible fuelmixture mixed in premix zone 144 exits through exit opening 138 andburns, preferably in a smokeless fashion, in the atmosphere.

The outer member of the flare tip units of the flare apparatus describedherein may comprise a number of different configurations. The upperportions of some exemplary configurations are shown in FIGS. 8-14. FIG.8 shows an outer member 150 with a convergent cone 152 extendingupwardly from the general cylindrical section 154 thereof. The coneangle 155 is between 0° and 75° and preferably roughly 17°. The exitopening 156 defined by convergent cone 152 preferably has an area notless than, and more preferably 20% more than the area of the choke point158 of the annular fuel passage which is essentially the annular gasoutlet. If desired, the upper end of the inner member of the flare tipunit can be fitted with a convergent cone 160 or divergent cone 162 asshown in FIGS. 9 and 10.

The outer member of the flare tip unit in FIG. 11 has first and secondconvergent cones 164 and 166 extending upwardly from the cylindricalportion 167 of the outer member of the flare tip unit wherein the coneangle 168 for first convergent cone 164 is less than the cone angle 170for the second convergent cone 166. In FIG. 12, generally cylindricalportion 171 of the outer member may have first and second convergentcones 172 and 174, respectively, wherein first cone angle 176 is greaterthan second cone angle 178. A hyperbolic shape 180 extends upwardly fromthe cylindrical section 182 of the outer member of the flare tip unitshown in FIG. 13. The simplest configuration of a flare tip unit isshown in FIG. 14, which simply has straight cylindrical inner and outermembers 184 and 186. It is understood that each of the flare tip unitsshown in FIGS. 8-14 will operate like the flare tip units 15 describedherein. FIGS. 8-14 are added simply to exemplify the differentconfigurations that are possible. The inner member in all cases ispreferably a straight cylinder from the inlet to the outlet thereof withan optional inlet bell to direct steam.

As discussed herein, the preferred embodiment of the flare tip unitscomprise flare tip unit 15, which has an outer member 34 and an innermember 32 wherein inner member 32 is substantially straight from theinlet 44 to the outlet 46 thereof. If desired, flare tip units may beutilized wherein the inner member has a bend therein as depicted inFIGS. 6 and 7. Therein, flare tip units 200 and 200 a, respectively, areshown. Flare tip unit 200 a is similar to flare tip 200 and so the sameidentifying numerals will be utilized for common parts with thesubscript “a.” Flare tip unit 200 a adds an additional steam injectionlocation, so the primary description will be with respect to flare tipunit 200.

Flare tip unit 200 has an inner member 202 and outer member 204. Innermember 202 defines a passageway 203 and receives air, and preferably airmoved by steam from a steam injector 206. Steam and air enter inlet 208of inner member 202. Steam and air pass through an outlet 210 of theinner member 202. Inner member 202 passes through a side of outer member204 and has a bend 211 therein from an inlet section 212 to a generallyvertical section 214. Gas is communicated into outer member 204 andpasses upwardly through an annular gas passage 216 defined betweenvertical portion 214 of inner member 202 and outer member 204. Verticalsection 214 and outer member 204 are coaxial and share longitudinalcentral axis 215. A premix zone 218 is defined between outlet 210 andthe exit opening 220 of outer member 214. Flare tip unit 200 a isidentical except that steam is injected into the inner member from adoughnut-shaped plenum 222 which has a plurality of openings 223 tocommunicate into the inner member 202.

The flare apparatus, whether used as a single flare tip unit or as aplurality of flare tip units with a single combustible gas supplyreduces the amount of steam necessary to achieve smokeless burning. Forexample, for a single flare tip unit comprising two straight cylinderslike that shown in FIG. 14, a steam consumption rate of 3,200 pounds anhour achieved smokeless combustion of 13,000 pounds per hour ofpropylene. The inner member was an 8-inch diameter tubular member andthe outer member was a 12-inch diameter tubular member. A similarlysized prior art apparatus similar to that shown in FIGS. 16 and 17, butwhich uses only center and upper steam injectors, requires 6,000 poundsper hour of steam to achieve smokeless burning of 16,000 pounds per hourof propylene. Thus, there is a 34% reduction of steam consumption. Whenthe single unit as described herein is mathematically scaled up by afactor of two to a 16-inch diameter inner member and a 24-inch diameterexternal member, and the premix zone modified to that in FIG. 15. 13,000pounds per hour of steam were required for 39,000 pounds per hour ofsmokeless combustion of propylene. For a similarly sized flare apparatuslike that shown in FIGS. 16 and 17, 16,000 pounds per hour of steam arerequired to achieve 34,500 pounds per hour of propylene which is a 28%reduction of steam for propylene. When a plurality of flare tip unitsare connected by a plenum, the improved efficiencies are similar tothose for single flare tip units, and in many cases may be higherbecause the space between the multiple flare tip units 15 allows airfrom the atmosphere to be entrained into the individual flames from eachflare tip unit. Each individual flare tip unit has a flame thereaboveand at some point all of the flames will merge to form a generallycylindrical flame with a hollow interior. Air may be entrained into themerged flames from the hollow interior. Ultimately as the height of theflame grows, a single flame may exist. Because of the additional airentrainment into the flame from the atmosphere, the current invention ismore efficient in terms of smokeless performance than the prior artconfiguration which comprises a single flame as it exits the flare tipand will therefore entrain less air from the atmosphere than the currentinvention.

Thus it is seen that the present invention is well adapted to carry outthe objects and attain the ends and advantages mentioned above as wellas those inherent therein. While certain preferred embodiments of theinvention have been described for the purpose of this disclosure,numerous changes in the construction and arrangement of parts and theperformance of steps can be made by those skilled in the art, whichchanges are encompassed within the scope and spirit of this invention asdefined by the appended claims.

1. A flare apparatus comprising: a plurality of flare structures forburning a combustible gas, comprising: an outer member having first andsecond ends, the second end defining an exit opening; and an innermember having an inlet and an outlet, at least a portion of the innermember being disposed in the outer member to define an annular gaspassage therebetween, the inner member outlet being positioned below theexit opening of the outer member; a plenum for receiving the combustiblegas from a combustible gas source and communicating the combustible gasto the annular gas passages in the flare structures, the inner memberextending from an exterior of the plenum through a plenum wall and intothe outer member, the inner member having a substantially straightlongitudinal central axis from the plenum wall through which it passesto the inner member outlet; and steam injectors for injecting steam intothe inner member inlet of each inner member at a rate sufficient to drawair from the atmosphere outside the plenum into the inner member, andmove the air through the inner member wherein an air/steam mixture exitsthe inner member outlet into the outer member and the air/steam mixturemixes with the combustible gas from the annular gas passage in the outermember above the inner member outlet, and the combustible gas andair/steam mixture exits the outer member through the exit opening forburning, wherein the exit openings of each flare structure are spacedapart such that each flare structure has an individual flame thereabove,and wherein air is entrained into each individual flame.
 2. The flareapparatus of claim 1, wherein the first end of the outer member of eachof the flare structures defines a gas inlet for communicating thecombustible gas into the annular gas passage.
 3. The flare apparatus ofclaim 1, the inner member of each of the flare structures comprising astraight cylinder from the inner member inlet to the inner memberoutlet, wherein the inner member passes completely through the plenum.4. The flare apparatus of claim 1, the outer member of each flarestructure comprising: a cylindrical portion having an upper end; and aconvergent cone extending upwardly from the upper end of the cylindricalportion, wherein the convergent cone directs combustible gas from theannular gas passage radially inwardly toward the air/steam mixtureexiting the inner member outlet.
 5. The flare apparatus of claim 4,wherein at least a portion of the convergent cone extends above theoutlet of the inner member.
 6. The flare apparatus of claim 4, theportion of the inner member passing through the plenum comprising astraight cylinder.
 7. The flare apparatus of claim 1, wherein the exitopenings are spaced such that the flames from each flare structure mergeinto a flame with a hollow interior.
 8. A flare apparatus for burning acombustible gas comprising: a plurality of flare tip units, each flaretip unit comprising: a single outer member having first and second ends,a portion of the single outer member defining a convergent cone; and asingle inner member having an inlet and an outlet, wherein at least aportion of the inner member is disposed in the outer member to define anannular gas passage and wherein at least a portion of the convergentcone extends above the inner member outlet; and a plenum connectable toa source of combustible gas for communicating the combustible gas to theannular gas passage of each of the flare tip units, wherein all of thecombustible gas received by the plenum is communicated to the annulargas passages of the flare tip units and each annular gas passagereceives a portion of the combustible gas, the combustible gas exitingthe annular gas passage in each flare tip unit being directed radiallyinwardly by the convergent cone to mix in the outer member with at leastair passing through the outlet of each inner member, and wherein the atleast air and combustible gas mixture passes through an exit openingdefined at the second end of the outer member and is burned to createthe individual flame thereabove.
 9. The flare apparatus of claim 8, theplenum defining a plenum interior, combustible gas being communicatedinto the plenum interior from the combustible gas source, wherein thefirst end of each outer member is located within the plenum interior andextends from the plenum interior to a plenum exterior, and wherein theinner member passes completely through the plenum, so that the innermember inlet and outlet of each flare tip unit are outside the plenum.10. The flare apparatus of claim 9, the inner member comprising astraight cylinder from the inner member inlet to the inner memberoutlet.
 11. The flare apparatus of claim 8, the plenum comprising acurved upper plate connected to a curved lower plate to define theplenum interior.
 12. The flare apparatus of claim 11, wherein the curvedupper and lower plates are connected by a side wall.
 13. The flareapparatus of claim 9, wherein each outer member comprises: a firstcylindrical portion, the convergent cone extending upwardly from thefirst cylindrical portion.
 14. The flare apparatus of claim 9, furthercomprising a steam injector associated with each inner member forinjecting steam into the inner members of each flare tip unit at a ratesufficient to draw air into the inner member and move air therethrough.15. The flare apparatus of claim 14, each flare tip unit defining apremix zone between the inner member outlet and the exit opening inwhich a steam/air mixture exiting the inner member outlet mixes withcombustible gas exiting the annular gas passage prior to thegas/steam/air mixture passing through the exit opening.
 16. The flareapparatus of claim 14, wherein the steam injectors receive steam from asingle steam source.
 17. A flare apparatus comprising: a plurality ofsteam-assisted flare structures for burning a combustible gas, eachflare structure comprising: a single outer member having first andsecond ends, the second end defining an exit opening; a single innermember having an inner member inlet and an inner member outlet, at leasta portion of the single inner member being disposed in the outer memberand defining a passage therethrough for air and steam, the inner memberhaving a straight longitudinal central axis from the inner member inletto the inner member outlet; an annular gas passage defined by andbetween the outer member and the inner member; and a steam injector forinjecting steam into the single inner tubular member at a ratesufficient to pull air from outside the inner and outer members into theinner member, wherein steam and air pass through the inner member outletinto a premix zone in the outer member and wherein gas communicated intothe annular gas passage exits the annular gas passage into the premixzone, and wherein the convergent cone defines at least a portion of thepremix zone and directs gas from the annular gas passage radiallyinwardly in the premix zone where gas mixes with the steam and airtherein, so that a combustible gas, steam and air mixture exits throughthe exit opening; and a plenum, wherein the plenum receives thecombustible gas from a combustible gas source and communicates thecombustible gas into the annular gas passages of the plurality of flarestructures, and wherein the inner member passes completely through theplenum so that the inner member inlets and inner member outlets of theflare structures are positioned exterior to the plenum.
 18. The flareapparatus of claim 17 wherein a single steam source provides steam toeach of the plurality of steam injectors in the plurality of flarestructures.
 19. The flare apparatus of claim 17 wherein the first end ofeach outer member defines a gas inlet to the annular gas passage in eachof the plurality of flare structures.
 20. The flare apparatus of claim17 wherein the portion of the inner member disposed in the outer memberis a straight cylinder.
 21. The flare apparatus of claim 17 wherein theexit openings are spaced apart so that each has an individual flamethereabove.
 22. The flare apparatus of claim 21, wherein the exitopenings are spaced so that the individual flames from each of the flarestructures merge into a flame with a hollow interior.